A wireless sensor network (WSN) (hereinafter, also referred to simply as a “sensor network”) is a communication network including a wireless terminal (which is referred to as a “sensor node”) having a plurality of sensors and is used to collect information from each sensor node. In the WSN, a communication scheme such as ZigBee®, EnOcean®, Wi-SUN®, or Bluetooth® low energy (BLE) is used.
Data transmitted from each sensor node is relayed by a relay, and then transmitted to a higher calculator such as a server or the like. The higher calculator executes various processing based on data received from a plurality of sensor nodes.
In the sensor network, it is preferred that each sensor node can operate without requiring an external power source. Thus, generally, each sensor node is configured to operate intermittently to consume less power. The intermittent operation refers to driving a peripheral device such as a sensor and a communication device only when a task is executed.
Meanwhile, in order to detect abnormality of a monitoring target, a plurality or sensors are required to operate constantly. In a situation where the plurality of sensors constantly operate in this way, a problem of securing a communication power in use, a problem of securing a radio band to transmit a huge amount of data, and the like may arise. Thus, a reduction in an amount of data to be transmitted is an essential task.
In the related art, a sensor network system is provided which is also capable of securing a communication band of a wireless network, while having a plurality of sensor nodes for measuring data having a high sampling rate. Specifically, a sensor node in the related art calculates a feature quantity from an observation value obtained during a predetermined observation period and determines whether the feature quantity thus calculated exceeds a predetermined threshold value. The sensor node transmits the observation value obtained during the predetermined observation period to a server only when the feature quantity exceeds the predetermined threshold value. Here, the feature quantity refers to a quantity obtained by digitizing features of the observation value, and also to a quantity that may be used as a standard of judgment. Single data or a plurality of data having a data amount smaller than a plurality of observation values are calculated using the plurality of observation values and used as feature quantities in the related art.
In the related art mentioned above presents only an example as to how a feature quantity is actually calculated. Specifically, in the related art, the sensor node includes a 3-axis accelerometer as a vibration sensor for analyzing stress of a structure, and a power value corresponding to a first natural frequency in a power spectrum is used as a feature quantity. However, in an actual calculation, a 2-square average value of acceleration of the original temporal signal (i.e., of the entire frequency domain) is simply calculated as an approximate value in the related art. As mentioned above, in the related art, since only the power value (actually, an overall power value of a predetermined period) at the first natural frequency is utilized as a feature quantity, the method is not considered to be appropriate for a case where another feature is more important, for example, in a case where it is intended to focus on a change in the power value over time at a predetermined frequency (a high-order natural frequency or the like).
Further, in the related art, it is difficult for a feature quantity to exceed a threshold value due to a short waveform or the like for an observation period, causing a difficulty in detecting a waveform. Since abnormal vibrations or the like have a pulse shape in many cases, a transmission period should be set shorter in order to detect such vibrations. However, there is a possibility of increasing communication frequency and generating communication interference with another sensor and this also makes it difficult to secure an intended radio band. Moreover, when the feature quantity frequently exceeds the threshold value, it is difficult to acquire the effect of reducing a communication amount. As another method to solve this problem, a sensor node that frequently performs a threshold value determination is considered. However, since a state such as chattering (a state where a feature quantity frequently exceeds a threshold value) occurs depending on signals, there is a possibility of performing high frequency transmission, a great amount of electric power is consumed in the sensor node and a communication failure occurs. As a result, it is not suitable for a wireless sensor node.